This invention relates to a supported rhodium catalyst and a method of preparing same. The present invention is also directed to a process for the production of acetic acid from methanol and carbon monoxide using a supported rhodium catalyst.
U.S. Pat. No. 5,155,261 discloses a process for producing acetic acid by reacting methanol in a solvent with carbon monoxide in the presence of an alkyl iodide and a solid catalyst containing a rhodium complex supported on a porous, cross-linked vinyl pyridine resin carrier and suggests the preferability of using Reilex-425 (product of Reilly Tar and chemical Corporation) as the vinyl pyridine resin. The vinyl pyridine resin has a degree of cross-linking of 33% and a pore volume of 0.71 cc/g. The catalyst disclosed in this prior art has a satisfactory carbonylation activity.
However, it has now been found that the catalyst is ill-suited for the industrial production of acetic acid because of poor resistance to abrasion and resistance to elimination of pyridine rings thereof. Namely, the vinyl pyridine resin is not only gradually chemically decomposed but also gradually abraded or pulverized as the carbonylation is continued for a long period of time. The elimination of the pyridine rings makes it impossible to keep the catalytic activity constant and causes reduction of the catalyst life. The pulverization of the catalyst not only causes the reduction of the catalyst life but also requires the separation of the pulverized powder from the carbonylation reaction product.
Hitherto, supported rhodium catalyst has been prepared by contacting a pyridine-containing polymer with an organic solvent solution containing an alkyl iodide and a rhodium salt under a pressure of carbon monoxide. Since the solubility of a rhodium salt, such as RhI.sub.3, in an organic solvent is very low, the rhodium salt is apt to be precipitated during the preparation of the catalyst. Thus, when the catalyst thus produced is separated from the reaction mixture, part of the precipitated rhodium salt is unavoidably admixed into the catalyst. This causes loss of very expensive rhodium. Further, another part of the precipitated rhodium deposits onto the inside wall of the reactor, which also causes loss of rhodium. The precipitation of the rhodium salt could be prevented by use of a large amount of the organic solvent. However, this is disadvantageous from the standpoint of economy not only because the additional cost of the organic solvent but also because the necessity of use of a large reactor.